1. Field of the Invention
Powder-form, granular or tableted detergents for dishwashing machines consist essentially of pentasodium triphosphate, alkali silicate, mainly alkali metasilicate, and optionally a low-foaming, preferably chlorine-stable, nonionic surfactant and bleaches, preferably those which release active chlorine in aqueous solution. They are used above all to facilitate the removal of tea, coffee or fruit juice stains.
Because of their high content of alkali silicates and pentasodium triphosphate, the detergents are strongly alkaline, their in-use solutions having a pH value of from about 11 to 12. Sodium hydroxide is not added because, in view of the normal contact times in domestic dishwashing machines, porcelain, glazed-on decorative patterns and glasses would be affected by the high alkalinity. This phenomenon is known as corrosion. By contrast, silicate synergistically supports the detergent effect of the triphosphate and inhibits corrosion in certain concentrations in the triphosphate/silicate mixture.
Known detergents of the type in question are generally produced by standard methods, such as mixing or granulation. In powder form or granulate form, they are also used in the usual concentrations, i.e. in quantities of from about 30 to 50 g per machine load.
2. Description of the Prior Art
Powder-form or granular detergents of the type in question are described, for example, in British Pat. No. 1,442,885 and in U.S. Pat. Nos. 3,816,320, 3,936,386, 3,933,670, and 4,083,795.
U.S. Pat. No. 3,390,092 discloses disc-like tablets which are produced from simple mixtures of known detergent ingredients using a tabletting aid.
Tablets of the type mentioned above are also described in U.S. Pat. No. 4,219,436. The content of alkaline components (hereinafter referred to as active substance [AS]), including the proportion of alkali silicates and pentaalkalitriphosphates, is of the order of 65 to 95% by weight, based on the tablets as a whole. Accordingly, detergent tablets such as these based on compressed raw materials fully correspond in their composition to powder-form or granular detergents.
The disadvantage of producing a tablet based on compressed individual components is that mechanical strength is inverse to the dissolving rate. Because of this tabletting aids and so-called disintegrating agents have to be added. Their function is, on the one hand, to ensure homogeneity of the powder-form or granulated raw materials during the compression molding operation and, on the other hand, to facilitate disintegration of the tablet on contact with water by swelling of the disintegrating agent, so that better solubility is obtained. However, neither additive is of any value to the washing process as such, in other words, the additives in question reprsent ballast and, in addition, take time to incorporate in the detergent.
U.S. Pat. No. 2,412,819 describes briquetted detergents for dishwashing machines which are produced simply by mixing together all the alkaline-reacting active substances, such as for example up to 65% by weight, based on the detergent as a whole, of sodium silicates and pentasodium triphosphate and, if necessary, water if the preferred water of hydration of the compounds mentioned is not sufficient, and then gently heating the mixture with stirring to 90.degree.-100.degree. C. until a uniformly molten mass is obtained, subsequently pouring the melt thus formed into molds and leaving it to solidify into a compact crystal aggregate. No additions of active chlorine donors are mentioned in U.S. Pat. No. 2,412,819. This is understandable because the detergents themselves are strongly alkaline and, like many substances which it would be desirable to add, but which are sensitive to alkali, the active chlorine donors would be inactivated during the actual fusion process.
Detergents in the form of fused blocks for dishwashing machines are also described in European Pat. No. 3,769. In most cases, the detergents described contain large amounts of alkali hydroxides. However, Example 8 discloses a composition which is free from alkali hydroxides and although, in that example, the active chlorine donor is stirred directly into the subsequently solidifying aqueous solution of the ingredients, the chlorine donor is generally added in the form of a separate core. In this example, the AS-content is only 60% by weight, based on the detergent as a whole, which is too low for use in domestic dishwashing machines. Since the patent specification repeatedly mentions, even in comparison tests, the well known sensitivity of active chlorine donors to alkalis, it can not be assumed that active chlorine donors can be directly incorporated into strongly alkaline block-form detergents free from alkali hydroxides.